Zpass channel "absolute"(not relative to distance from camera)

hi!
im compositing my renders in AE and using the zpass to do compositing based in the "luma inverted"matte.
it works fine for still images because the blender camera is in a fixed position.
But if you move the camera in along the scene , it doesn´t work because the distance from the camera changes.

is there any tecnique to render a zpass with the greyscale relative to the scene and not relative to the camera?

ok,my question is a mess:(, im sorry,i try my best.
thanks

ok! you point me in the right direction
thanks a lot